The present invention relates to a semiconductor device and a method for forming the same, more particularly, to a complementary-metal-oxide-silicon CMOS image sensor and a method for forming the same.
The CMOS image sensor is a device transforming an optical image into an electric signal. The CMOS image sensor has lower power consumption than the currently known charge coupled device CCD image sensor. In addition, the CMOS image sensor is advantageous for its high integration and simple driving method. Numerous research studies on CMOS image sensors have been prompted by the recent trends that require the rapid development of a CMOS manufacturing process of a semiconductor device.
In general, a pixel of a CMOS image sensor is divided into a light receiving unit and a CMOS unit for controlling signal charges accumulated in the light receiving unit. The light receiving unit may consist of photodiodes, and the CMOS unit may include some MOS transistors.
Concurrent with the high integration of a semiconductor device is the requirement for a CMOS image sensor to be operational at a high speed. In order to improve the operational speed of the CMOS image sensor, a method is used to reduce the resistance of a gate included in a MOS transistor. The gate may include tungsten silicide with a low resistivity in order to reduce the resistance of the gate. The technology that includes tungsten silicide in a gate of a MOS transistor in a pixel is disclosed in the Korean Laid-open Patent Publication No. 2000-41451. The method for forming a MOS transistor, as disclosed in the Korean Laid-open Patent Publication No. 2000-41451, is described next, with reference to the accompanying simplified drawings.
FIGS. 1 and 2 are cross-sectional views illustrating a method for forming a conventional CMOS image sensor.
Referring to FIGS. 1 and 2, a gate oxide layer 2 is formed on a substrate 1, and a polysilicon layer 3 and a tungsten silicide layer 4 are sequentially formed on the gate oxide layer 2.
The tungsten silicide layer 4, the polysilicon layer 3 and the gate oxide layer 2 are patterned in series to form a gate electrode of a transfer transistor. The gate electrode includes a polysilicon pattern 3a and a tungsten silicide pattern 4a, which are sequentially stacked.
Impurity ions are selectively injected into the substrate 1 at one side of the gate electrode to form a photodiode 5. Impurity ions are selectively injected into the other side of the gate electrode to form an impurity doping layer 6. Even if not shown in FIGS. 1 and 2, a spacer is formed at the sidewalls of the gate electrode.
The tungsten silicide pattern 4a has a lower resistivity than the polysilicon pattern 3a. In turn, the resistance of the gate electrode is reduced by the tungsten silicide pattern 4a, which results to an improved operational speed of a CMOS image sensor.
However, when various kinds of wet cleaning processes are performed on the substrate 1, both after the tungsten silicide pattern 4a is formed and before the spacer is formed, the sidewalls of the tungsten silicide pattern 4a are exposed. Consequently, the photodiode 5 may be contaminated at the exposed portions of the tungsten silicide pattern 4a. The photodiode 5 may increase dark signals due to the metallic contamination. A dark signal can be described as a leakage current due to signal charges generated from the photodiode 5 when an external light is shielded. Accordingly, as the dark signal is increased, the CMOS image sensor may transmit an erroneous image signal; or, alternately, the CMOS may transmit a distorted image signal when impinged by an external light.